Synchronous Motors

SYNCHRONOUS MOTORS

Synchronous Machine or Motor is electrically identical with an alternator or ac generator. A given synchronous machine may be used at least theoretically as an alternator, when driven mechanically or as a motor when driven electrically just as in the case of de machines. Synchronous motors are rated between 150 kW and 15 MW and run at speed ranging from 150 to 1800 rpm.

Some of the characteristics of synchronous motor are -

(1) It runs either at synchronous speed or not at all.

Synchronous Speed N_s = 120 f / p

(2) It is not inherently self starting. It has to be run upto synchronous speed by some means, before it can be synchronized to the supply.

(3) It is capable of being operated under a wide range of power factors, both lagging and leading. Hence it can be used for power correction purposes, in addition to supplying torque to drive loads.

Principle of Operation

When a 3φ winding is fed by a 3φ supply then a magnetic flux of constant magnitude but rotating at synchronous speed is produced. Consider a two pole stator rotating at synchronous speed in clockwise direction.

With the rotor position as shown in figure. Suppose the stator poles are at that instant situated at points A and B. The two similar poles, N and N_S as well as S and S_S will repel each other, with the result that the rotor tends to rotate in the anticlockwise direction.

But half a period later, stator poles having rotated around, interchange their position. i.e., N_S is at point B and S_S at point A. Under these conditions, N_S attracts S_S and attracts N. Hence rotor tends to rotate clockwise. Hence we find that due to continuous and rapid rotation of stator poles, the rotor is subjected to a torque which is rapidly reversing. For large inertia the rotor cannot instantaneously respond to such quickly reversing torque, with the result that it remains stationary.

Now consider the condition shown in Figure 3.53(a). The stator and rotor poles are attracting each other. Suppose that the rotor is not stationary, but is rotating clockwise, lo with such a speed that it turns through one pole-pitch by the time the stator poles interchange to their positions as shown in Figure 3.53(b). Here again the slator and rotor poles attract each other. It means that if the rotor poles also shift their positions along with the stator poles, then they will continuously experience a unidirectional torque i.e., Clockwise Torque.

Method of Starting

The rotor is speeded up to synchronous/near synchronous speed by some arrangement and then excited by the dc source. The moment this synchronously rotating rotor is excited, it is magnetically locked into position with the stator. It is because of this interlocking of stator and rotor poles that the motor has either to run synchronously or not at all. The synchronous speed is given by:

N_S = 120 f / P